In nuclear magnetic resonance (NMR) machines, a high-intensity magnetic field is generated by an extremely strong magnet, which usually is superconducting.
One conventional NMR main magnet design is toroidally shaped. An extremely strong, extremely uniform magnetic field is generated within a predetermined volume within an axial bore of this magnet. Inserted into the axial bore are the sample, tissue or body to be analyzed, and a combination radio frequency transmitting and sensor coil. The RF probe coil is situated to generate an oscillating field at right angles to the main field. The oscillating RF field causes an oscillation in the alignment of the chemical species. The oscillation of the chemical species within the magnet causes the emission of radio frequency signals, which are sensed by the RF probe coil.
In order to generate the appropriate frequency for oscillation, it is necessary to tune the inductive and capacitive elements of the sensor or probe coil such that there is optimum resonance in the circuit corresponding to the desired frequency. This tuning will vary according to the type of coil used and with the environment in which it is put. In general, tuning of the coil must occur with each new sample or body to be analyzed.
It is also necessary to match the impedance of the inputs of the probe coil to an RF-energy generating source in order to obtain the maximum transmission of RF-energy into the coil and to optimize the signal to noise ratio. Because the criteria for tuning and matching are different, the tuning and matching process is iterative, time consuming and difficult, as will be shown in more depth in the detailed description below.
In view of the foregoing, a need has arisen for methods and apparatus for tuning and matching an NMR probe coil wherein the resonant frequency and impedance matching conditions are decoupled from each other.